According to a Microsoft spokesperson, developers working on WP7 can unlock and use the phones with no extra cost. While this makes sense, it's surprising in that other phone developers (such as Android and Apple) have to purchase the rights to unlock the phones.
Developers can unlock up to five units and dispose of the phone's security walls, making WP7 a little more attractive to them than perhaps other platforms. In addition, this will include side-loading of applications and running unsigned applications, something which consumers are not allowed to do.
Such methods are required for developers but also lead to advanced system modifications like on the iPhone ("jailbreaking") or flat out custom ROMs like Android ("rooting"). It's not clear what will happen with WP7 at this point, but holes in the system can be exploited--that's just a fact.